1. Field of the Invention
The present invention relates to a coupling lens which can be used for a compatible optical disc apparatus that is compatible with different types of optical recording media, such as a compact disc (CD), a digital versatile disc (DVD), a High-Density DVD (HD-DVD) and a Blu-ray Disc, and which is a multi-wavelength optical system that uses a plurality of kinds of monochromatic light, and an optical pickup device using the coupling lens.
2. Description of Related Art
With a recent increase in the capacity and the density of optical discs, new-standard high density optical discs such as HD-DVD and Blu-ray Disc are proposed and put into practical use. Accordingly, there is a demand for a compatible optical disc apparatus which permits recording and playback of different kinds of optical discs including the new-standard high density optical discs as well as CD and DVD.
In order to enable recording and playback of a high density optical disc with an improved recording capacity, it is necessary to reduce the diameter of an optical spot that is obtained by an optical system of an optical pickup device which is used for an optical disc apparatus. Because the spot diameter is proportional to λ/NA (where λ is a wavelength of a light source and NA is a numerical aperture of an objective lens), the spot diameter can be reduced by shortening the wavelength or increasing the numerical aperture. As for the shortening of the wavelength, a blue-violet semiconductor laser with a wavelength of about 400 nm has been studied and put into practice.
Generally in an optical pickup device, a laser power for recording on an optical disc is higher than a laser power for playing back an optical disc. Therefore, a phenomenon that a wavelength of light increases by several nanometers, which is called mode hopping, occurs upon switching from playback to recording. With the mode hopping, a focus position shifts. Although the focus shift can be eliminated by auto-focusing an objective lens, the focus shift increases if a wavelength of light is as short as about 400 nm. Accordingly, a recording error due to the focus shift occurs during the period of several nanoseconds until the auto-focus is locked in. It is thereby necessary to provide correction for reducing the focus shift or chromatic aberration due to the mode hopping.
Regarding this, Japanese Unexamined Patent Application Publication No. 2002-303788 discloses a lens which is designed to correct chromatic aberration by the diffraction of a diffraction pattern. Further, Japanese Unexamined Patent Application Publication No. 2004-185746 discloses a coupling lens which is designed to correct chromatic aberration without deteriorating light use efficiency by a method that does not use the diffraction.
However, the method of using the diffraction which is disclosed in Japanese Unexamined Patent Application Publication No. 2002-303788 increases a total number of annular zones and it causes the degradation of diffraction efficiency or light use efficiency. Further, the coupling lens for chromatic aberration correction which is disclosed in Japanese Unexamined Patent Application Publication No. 2004-185746 is intended for the exclusive use of a blue-violet semiconductor laser with a wavelength of about 400 nm. It cannot be used as a compatible objective lens which enables the use of two or more wavelengths including a wavelength for CD (about 780 nm), a wavelength for DVD (about 650 nm) and so on. It therefore fails to reduce the size of an optical pickup device by using a three-wavelength compatible objective lens which is compatible with all of CD, DVD and the above-described new-standard high density optical discs so as to have an optical system in common for the three wavelengths.
In addition, in the case where an optical system is used in common for three wavelengths so as to be compatible with all of CD, DVD and the new-standard high density optical discs, if a step height in a annular zone of the coupling lens for chromatic aberration correction is set by placing a high priority on the correction of chromatic aberration at a laser wavelength for a high density optical disc which has the shortest wavelength, incident light to an objective lens becomes finite at a laser wavelength for CD. Thus, coma aberration which occurs when an objective lens shifts during tracking becomes a problem.